The invention relates to a sandwich structure for vehicle bodies, in particular for flat areas of a vehicle including which require sufficient rigidity and good thermal and sonic insulation.
In order to achieve sufficient rigidity in the case of prior art construction of a roof (GM 79 29 367 U1), an initially separate semi-finished profile frame, rigid with respect to torsion, must first be prepared and must then be irremovably fastened to the outer skin of the roof along its edges. The foamed on plastic layer covers the semi-finished product profile frame connected irremovably to the skin of the roof. If the skin of the roof consisting of light metal or plastic is to be effectively made rigid with respect to bending, according to the known proposal, at least one profile strut extending transversely to the longitudinal beams of the semi-finished product profile frame must be provided. The prior art construction of such a roof requires a significant expenditure in material and production.
The corresponding description applies to the sandwich structures for the bodies of vehicles known from GB 2 311 966 A. According to this state of the art, an outer shell and an inner shell for the sandwich structure are produced from plastic by means of injection molding, and connected firmly and tightly to one another in a subsequent step by a plastic welding process or by the application of adhesive along their edges. Then the hollow body so formed is filled with plastic and allowed to harden. Alternatively, a filling body can be prepared in advance which is inserted between the outer and the inner shells when they are connected together. For higher rigidity requirements, reinforcing parts of aluminum must be inserted in the cavity between the outer and inner shells provided the outer shell is formed with channels for accommodating such reinforcing parts.
Finally, roofs of composite material are also known (DE 32 02 594 C2) that are composed of an airtight and rain-tight outer skin, a central honeycombed/structural layer with aluminum or cardboard webs or moderately hard foam and fleece structures, a partly rigid, porous inner layer, and a cushion and/or decorative layer, and produced by means of a hot pressing process.
In general terms, this invention includes a sandwich structure for a vehicle body. The structure has a deep-drawn outer skin and, foamed onto the inner side of the outer skin, a plastic layer which is provided, if necessary, with a textile flat pattern or a decorative plastic film on its upper surface, the outer skin is formed without a frame at its outer edges, and is provided with an encircling canting where the foamed-on plastic layer reaches up to the canting, and in the foamed-on plastic layer a sheathingxe2x80x94not rigid in itselfxe2x80x94is provided over the entire flat area of the outer skin by which method the modulus of elasticity of the foamed-on plastic layer is increased.
According to the invention, a sandwich structure for the body of a vehicle is provided without additional expensive profile elements for frames and struts, but which nonetheless, due to the effect of the outer skin and sheathed plastic layer, achieves a rigidity with respect to bending and total rigidity that satisfies all the stability requirements of the sandwich structure for the operational demands of the vehicle. The sheathing is not rigid in itselfxe2x80x94set within the plastic layer makes possible an effective sheathing of the plastic layer up to into the edge areas of the outer skin when formed with an encircling canting in a certain way.
Textiles, knitted materials, fleece, grids, mats, and so on such as glass, plastic (for example, polyester or aramide), and/or carbon fibers which are bendable or not rigid in themselves come into consideration as sheathing to the extent that they are adapted to the form of the outer skin into their edge areas, and can be penetrated and encircled by the foamed-on plastic in the closed foaming tool. Preferably, however, fibrous materials are used for the formation of the sheathing that are incorporated into the plastic layer as sections of fiber lengths in uniform distribution but in unordered position.
Particularly good results were achieved by the use of a sheathing of fiberglass materials that are present in longitudinal sections of twisted fiberglass bundles of ca. 4,800 TEX between ca. 12 mm and ca 100 mm in length. The modulus of elasticity of the foamed plastic material, which has a value of ca. 300 N/mm2 without this sheathing, could be increased to over 1600 N/mm2 according to claim 4 with the application of a fiberglass sheathing of 25 wt % with respect to the total weight of the sheathed plastic foam.
The fiberglass materials can be used in longitudinal sections of different lengths where particularly good sheathing results have been achieved by the use of two different lengths with a larger percentage on the smaller length, about two thirds of the amount of fiberglass. In the scope of the range of longitudinal sections, however, longitudinal sections of a single length can also be used.
Metallic materials such as steel or aluminum sheets are suitable for the outer skin. However, the use of an outer skin of thermoplastic plastic film, deep-drawn in a vacuum, preferably of a two-layer co-extrusion film, yields a sandwich structure of convincing stability.
The co-extrusion film advantageously includes PMMA for the outer layer as well as a mixture of PC and ASA for the inner layer, and has the layer thickness ratios specified whereby the thickness of the outer layer preferably makes up about 15% of the total thickness of the outer skin. For example, the total thickness of the co-extrusion film is about 1.3 mm of which about 0.2 mm is due to the outer layer.
The use of a plastic also offers the advantage that the plastic in the mass can be prepared in the color desired such that the sandwich structure does not have to be retroactively lacquered. The plastic film can furthermore be given a shining surface by use of a high-gloss, polished, deep-drawn form in the form of a shell or a grained surface by previous calendering during the production of the outer skin.
If the outer skin is formed of a thin aluminum sheet, its thickness can be relatively small because of the sheathed plastic layer, preferably ca. 0.6 mm in the interest of conserving weight. For the outer skin the aluminum alloy 0.4 Si 1.2 is particularly suitable.
With the use of an aluminum sheet as material for the deep-drawn outer skin, the sheet can be lacquered ready-made in the desired color through its hydraulic deep-drawn deformation, and provided with a protective film. In the case of the deep-drawn deformation, the lacquer layer remains undamaged after withdrawal of the protective film. The lacquer structure can be multi-layered to withstand the deformation of the aluminum sheet with tears or other damage.
A covering polyester-based lacquer of about 18 to 23 um in thickness can be applied, which in turn can be covered by a clear lacquer layer of PVDF of 22 um thickness to a primary layer of epoxy resin of about 5 to 7 um in thickness a covering lacquer on the basis of polyester of about 18 to 23 um in thickness. The burning in temperature for a multi-layer lacquer structure of this type is about 240xc2x0 C. The hydraulic deep-drawn deformation of the ready made lacquered aluminum sheet can be done in several steps, for example in three steps. Deep-drawing rates of 60 mm/s have proven themselves as non-damaging for the lacquer structure.
Canting the outer skin before trimming the edges passes into an encircling flange edge angled outwards such that the sheathed plastic layer is guided on the canting up to the flange edge. The trimming of the edge of the sandwich structure, for example by contour cutting or laser section, removes the encircling, angled off flange edge and the outer area of the canting of the outer skin, as well as the sheathed plastic layer adhering to the areas sloping away near it. A canting edge of the outer skin, important for rigidity, and the sheathed plastic layer on the canting edge remains after the trimming. The edge section is guided through the canting as well as through the sheathed plastic layer, forming a section face which, in the case of the vehicle roof, could be set directly on the roof frame located on the vehicle body.
A variant for the encircling canting of the outer skin in which the canting passes into an encircling flange edge angled outward on foaming in of the plastic layer, a seal is applied to the inner side of the outer skin over its entire width to seal the foam form between the lower part and the upper part. In the case of this formation variant, trimming of the edge after the foaming process is entirely omitted. Moreover, the encircling flange edge angled outward can be stamped or fine-cut for the formation of a defined final geometry of the outer skin during deep-drawing of the outer skin or in a subsequent step. With flat application to a seal of the foam form, the encircling flange edge advantageously provides for a good and clean seal of the foam form during the following foaming process. In fact, it is possible in principle that only the outer skin has an encircling canting before the foaming process without an encircling flange edge angled outward that has already been removed, if necessary. However, the sealing of the foam form is then more difficult in structure, in particular the sharp cant of the final canting could have a negative influence on the lifetime of the seal of the foam form.
In a particularly advantageous development of the sandwich structure, there is banded onto the canting of the deep- drawn outer skin, a seal which has a two-legged, essentially U-shaped cross-section, where in the inner leg of the seal is foamed on the plastic layer while the outer leg forms the sealing surface. As a result, the sandwich structure is already provided in an advantageous manner. The plastic layer on the inner side of the outer skin is foamed on with an integrated seal that seals the sandwich structure with respect to the bordering components of the body without additional operational steps. For example, the banding of an additional, initially separately prepared seal onto the finished sandwich structure is necessary for this purpose; which provides additional cost savings. This development offers still another advantage in that during the foaming process the seal bond between the lower part and the upper part of the foam form, such that at this point in the process, no additional sealant has to be provided which would potentially wear by repeated use.
The plastic layer includes a PUR foam that is formed of a polyol and an isocyanate in a specified mixture ratio, about 1 to 2. The mixture of the two plastic components with the fiberglass sheathing is done according to the prior-art LFI process (long fiber injection).
The liquid plastic/fiberglass mass mixed in the mixing head is then applied to the deep-drawn outer skin laid on the lower part of the foaming tool, with its inner side pointing outward, approximately in its center,. beginning over an approximately path before the upper part of the tool is brought into the position above the lower part of the tool corresponding to the intended thickness of the foamed plastic layer. The driven foam fills the form and penetrates into the edge areas of the outer skin, up to the encircling canting of the outer skin. If necessary, the foam may be trimmed later.
If a plastic film is used as the outer skin, the inner side of the outer skin is expediently exposed to flame for activation before the application of the foamable plastic, then an outstanding bonding of the outer skin to the foamed plastic layer is achieved.
In the case where a decorative flat pattern is to be provided on the surface of the foamed on plastic layer, a pattern can be selected. The form of foam applied to the side of the plastic layer away from the outer skin such that a firm bond between the foamed on plastic layer and the decorative flat pattern results. For expediency, the decorative flat pattern may be temporarily stretched on the upper part of the foam tool, would be carried along by this into the foam position, and would remain on the plastic layer after the removal of the upper part of the tool.
A porous decorative flat pattern can be structured in multiple layers, where in the flat pattern is provided with a protective layer turned toward the plastic layer that prevents the penetration of the foamed-on plastic into its layer region through the pores of the decorative flat pattern.
Even in the foaming process, the sandwich structure can be utilized in an advantageous manner with additional function elements. Thus, a sandwich structure, can be provide for example in a vehicle roof whose outer skin has an opening on the side of the plastic layer. A guide profile for any optionally closing sliding cover where the opening is foamed on.
A sound absorbing surface layer structure can be foamed behind the plastic layer away from the outer skin, even during the foaming process, whereby the sound insulating characteristics of the sandwich structure can be improved still further for the formation of, as an example, hoods for a motor. Finally, instead of or in addition to the above measures relating to the macrogeometry of the surface of the plastic layer, a structuring of the microgeometry of the side of the sandwich structure turned away from the outer skin said structuring corresponding to the current sound-insulating requirementsxe2x80x94can also be done. According to which partially damaging regions with reduced density of the foam material are foamed on the side of the plastic layer away from the outer skin. This foaming of the damping regions on the plastic layer is done expediently in a second shot; that is, after foaming of the plastic layer on the inner side of the outer skin in a subsequent step, the damping regions are foamed on the plastic layer.